Fibronectin type III repeat based protein scaffolds with alternative binding surfaces

ABSTRACT

Protein scaffolds and scaffold libraries based on a fibronectin type III (FN3) repeat with an alternative binding surface design, isolated nucleic acids encoding the protein scaffolds, vectors, host cells, and methods of making thereof are useful in the generation of therapeutic molecules and treatment and diagnosis of diseases and disorders.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 13/628,393, filed 27 Sep. 2012, now U.S. Pat. No. 9,200,273, granted 1 Dec. 2015, which claims priority to U.S. Provisional Application Ser. No. 61/539,670, filed 27 Sep. 2011, the entire contents of which are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to protein scaffolds and scaffold libraries based on a fibronectin type III (FN3) repeat with alternative binding surface designs. More particularly, the present invention is directed to FN3 scaffolds and libraries having concave binding sites formed by select beta-strands and loops.

BACKGROUND OF THE INVENTION

Monoclonal antibodies are the most widely used class of therapeutic proteins when high affinity and specificity for a target molecule are desired. However, non-antibody proteins having relatively defined three-dimensional structures that can be engineered to bind desired target molecules, commonly referred to as protein scaffolds, may have advantages over traditional antibodies due to their small size, lack of disulphide bonds, high stability, and ability to be expressed in prokaryotic hosts. These scaffolds typically contain one or more regions which are amenable to specific or random sequence variation, and such sequence randomization is often carried out to produce libraries of proteins from which desired products may be selected. Novel methods of purification are readily applied; scaffolds are easily conjugated to drugs/toxins, penetrate efficiently into tissues and can be formatted into multispecific binders (Binz and Pluckthun, Curr Opin Biotechnol, 16, 459-469, 2005; Skerra, J Mol Recognit, 13, 167-187, 2000).

One such protein scaffold is the fibronectin type III (FN3) domain identified in a multitude of proteins, having a characteristic tertiary structure with 6 loops connected by 7 beta strands. Three loops in particular, the FG, BC, and DE loops are structurally analogous to the complementarity determining regions (CDRs) of antibodies. These loops have been randomized to generate libraries of the FN3 domain scaffolds to successfully select specific binders to a number of different targets while retaining important biophysical properties (Getmanova et al., Chem Biol, 13, 549-556, 2006; Hackel et al., J Mol Biol, 381, 1238-1252, 2008; Karatan et al., Chem Biol, 11, 835-844, 2004; Koide et al., J Mol Biol, 284, 1141-1151, 1998; Koide et al., Proc Natl Acad Sci USA, 104, 6632-6637, 2007; Parker et al., Protein Eng Des Sel, 18, 435-444, 2005; Xu et al., Chemistry & Biology, 9, 933-942, 2002). Libraries of the FN3 domains have been generated by randomizing also the AB, EF and CD loops (U.S. Pat. Pub. No. 2011/0038866; Int. Pat. Pub. No. WO2011/05133; U.S. Pat. Pub. No. 2011/0124527). Other references for FN3 libraries include Int. Pat. Pub. Nos. WO2002/32925, WO2003/104418, WO2009/023184 and WO2010/060095. Int. Pat. Pub. No. WO2012/016245 describes FN3 domain libraries using CD and FG loops together with surface exposed residues of the beta-sheet.

It would be advantageous to obtain improved fibronectin domain scaffold proteins for both therapeutic and diagnostic purposes. The present disclosure provides such improved proteins.

SUMMARY OF THE INVENTION

One embodiment of the invention is a method of making a library of fibronectin module of type III (FN3) domains having a diversified C-CD-F-FG alternative surface formed by a C beta-strand, a CD loop, an F beta-strand, and an FG loop, comprising providing a reference FN3 domain polypeptide having the amino acid sequence at least 80% identical to that of SEQ ID NO: 27; introducing diversity into the reference FN3 domain polypeptide by mutating at least one C beta-strand residue and at least one F beta-strand residue to form the FN3 domain library having the diversified C-CD-F-FG alternative surface.

A library produced by the methods of the invention.

A method of obtaining a protein scaffold comprising a fibronectin module of type III (FN3) domain having a diversified C-CD-F-FG alternative that specifically binds to a target molecule, comprising contacting or panning the library of claim 11 with the target molecule and isolating a protein scaffold specifically binding to the target molecule with a predefined affinity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows ribbon diagrams of FN3 domain and antibody VH domain structures. Loops of the FN3 domain structurally analogous to CDRs are labeled.

FIGS. 2A-C show structural diagrams that enable comparison between 2A conventional FN3 libraries with randomized loops; 2B FN3 library with randomized C-CD-F-FG alternative surface (TCL14 library); 2C FN3 library with randomized A-AB-B-BC-E surface (TCL15 library). Positions randomized in these library designs are depicted as solid black in the ribbon diagrams.

FIG. 3 shows sequence alignment of the Tencon27 scaffold (SEQ ID NO: 27) and the TCL14 library (SEQ ID NO: 28) having randomized C-CD-F-FG alternative surface. The loop residues are boxed. The particular loop and beta-strand regions are indicated above the sequences.

FIG. 4 shows sequence alignment of the Tencon27 scaffold (SEQ ID NO: 27) and a TCL15 library having randomized A-AB-B-BC-E alternative surface (SEQ ID NO: 61). The loop residues are boxed. The particular loop and beta-strand regions are indicated above the sequences.

FIG. 5 shows a topology diagram of the library design on Tencon27 (SEQ ID NO: 27) with randomized C-CD-F-FG alternative surface (the TCN14 library). Beta-strands are depicted as arrows with residues of the strands that are hydrogen bonded to one another in the Tencon27 structure placed adjacently in the plot. Positions of residues randomized are depicted with ovals shaded in grey.

FIG. 6 shows a topology diagram of the library design on Tencon27 (SEQ ID NO: 27) with randomized A-AB-B-BC-E alternative surface (the TCL15 library). Beta-strands are depicted as arrows with residues of the strands that are hydrogen bonded to one another in the tencon structure placed adjacently in the plot. Positions of residues randomized are depicted with ovals shaded in grey.

FIG. 7 shows expected and observed amino acid distribution at randomized positions in the TCL14 library.

FIG. 8 shows sequence alignment of Tencon27 (SEQ ID NO:27), TCL14 (SEQ ID NO: 28), and the designed libraries on FN10 (SEQ ID NO:98), TN3 (SEQ ID NO:99), and Fibcon (SEQ ID NO:62) with randomized C-CD-F-FG alternative surfaces. Residue numbering is based on Tencon27 sequence. The loop residues are boxed. The particular loop and beta-strand regions are indicated above the sequences.

DETAILED DESCRIPTION OF THE INVENTION

The term “fibronectin module of type III (FN3) domain” as used herein refers to a domain occurring frequently in proteins including fibronectins, tenascin, intracellular cytoskeletal proteins, cytokine receptors and prokaryotic enzymes (Bork and Doolittle, Proc Nat Acad Sci USA 89, 8990-8994, 1992; Meinke et al., J Bacteriol 175, 1910-1918, 1993; Watanabe et al., J Biol Chem 265, 15659-15665, 1990). Exemplary FN3 domains (or modules) are the 15 different FN3 domains present in human tenascin C and the 15 different FN3 domains present in human fibronectin (FN). Individual FN3 domains are referred to by domain number and protein name, e.g., the 3^(rd) FN3 domain of tenascin (TN3), or the 10^(th) FN3 domain of fibronectin (FN10).

The term “reference FN3 domain” as used herein refers to a wild type or non-naturally occurring FN3 domain that is used as a template into which substitutions are made to generate protein scaffolds specifically binding to a target molecule.

The term “alternative surface” as used herein refers to a surface on a side of the FN3 domain comprising two or more beta strands, and at least one loop. Exemplary alternative surfaces are a C-CD-F-FG surface that is formed by amino acids in the C and the F beta-strands and the CD and the FG loops, and an A-AB-B-BC-E surface that is formed by amino acids in the A, B and E beta-strands and the BC loop.

The term “substituting” or “substituted” or “mutating” or “mutated” as used herein refers to altering, deleting of inserting one or more amino acids or nucleotides in a polypeptide or polynucleotide sequence to generate a variant of that sequence.

The term “randomizing” or “randomized” or “diversified” or “diversifying” as used herein refers to making at least one substitution, insertion or deletion in a polynucleotide or polypeptide sequence.

“Variant” as used herein refers to a polypeptide or a polynucleotide that differs from a reference polypeptide or a reference polynucleotide by one or more modifications for example, substitutions, insertions or deletions.

The term “specifically binds” or “specific binding” as used herein refers to the ability of the FN3 domain of the invention to bind to a target molecule with an affinity (Kd) of at least 1×10⁻⁶ M, and/or bind to a target molecule with an affinity that is at least ten fold greater than its affinity for a nonspecific antigen (for example BSA or casein) as measured by surface plasmon resonance.

The term “target molecule” as used herein refers to a protein, peptide, carbohydrate, lipid, and the like having an antigen or an epitope that is recognized by the FN3 domain of the invention. The target molecule may be naturally or non-naturally occurring.

The term “library” refers to a collection of variants. The library may be composed of polypeptide or polynucleotide variants.

The term “tenascin C” as used herein refers to human tenascin C having a sequence shown in GenBank Acc. No. NP_002151 and in SEQ ID NO: 57. Tenascin C has 15 tandem FN3 domains that have amino acid sequences shown in SEQ ID NOS: 1-15, respectively. The amino acid sequence of the 3^(rd) FN3 domain of tenascin C (TN3) is shown in SEQ ID NO: 3.

The term “stability” as used herein refers to the ability of a molecule to maintain a folded state under physiological conditions such that it retains at least one of its normal functional activities, for example, binding to a target molecule.

The present invention provides fibronectin module of type III (FN3) domains that specifically bind to a target molecule, and thus can be widely used in therapeutic and diagnostic applications. The invention is based on a discovery that an alternative surface on a side of the FN3 domain comprising two or more beta-strands and at least one loop can be randomized to generate and select for protein scaffolds specifically binding a target molecule with high affinity. Published FN3-based domain libraries have been generated by diversifying either the top or the bottom loops, areas that structurally resemble CDRs in antibody variable chains, providing curved binding surfaces. In this invention, high affinity binding molecules are selected from FN3 domain libraries displaying concave interaction surfaces generated by randomizing an alternative surface; thus likely increasing the number of epitopes and targets against which high affinity binding protein scaffolds can be selected. The present invention provides polynucleotides encoding the protein domains or complementary nucleic acids thereof, vectors, host cells, and methods of making and using them. The present invention provides methods of making libraries of FN3 domains, and libraries made by methods of the invention.

Fibronectin Type III Domain

Fibronectin Type III (FN3) domain (or module) is a prototypic repeat domain initially identified in fibronectin and now known to be present in various animal protein families including cell surface receptors, extracellular matrix proteins, enzymes, and muscle proteins. Structurally the FN3 domains have a topology very similar to that of immunoglobulin-like domains, except for the lack of disulfide bonds. As is known in the art, naturally occurring FN3 domains have a beta-sandwich structure having seven beta-strands, referred to as A, B, C, D, E, F, and G, linked by six loops, referred to as AB, BC, CD, DE, EF, and FG loops (Bork and Doolittle, Proc Natl Acad Sci USA 89, 8990-8992, 1992; U.S. Pat. No. 6,673,901). Three loops, the BC, DE and FG loops are at the top of the FN3 domain, and three, the AB, CD and EF loops at the bottom of the domain (FIG. 1). Table 1 shows several FN3 domain containing proteins, and the number of different FN3 domains associated with each protein. While FN3 domain conformations are highly conserved, the similarity between different domains at the amino acid level is quite low.

FN3 domains may be naturally or non-naturally occurring. Exemplary non-naturally occurring FN3 domains are a consensus FN3 domain designed based on an alignment of select FN3 domains present in a certain protein and incorporating the most conserved (frequent) amino acid at each position to generate the non-naturally occurring FN3 domain. For example, a non-naturally occurring FN3 domain is designed based on a consensus sequence of the 15 FN3 domains from human tenascin C, or based on a consensus sequence of the 15 FN3 domains from human fibronectin. These non-naturally occurring FN3 domains retain the typical topology of the FN3 domains, and can exhibit improved properties such as improved stability when compared to the wild type FN3 domains. Exemplary non-naturally occurring FN3 domains are the Tencon and the Fibcon domains shown in SEQ ID NOS: 16 and 58, respectively, and described in U.S. Pat. Pub. No. 2010/0216708 and U.S. Pat. Pub. No. 2010/0255056.

TABLE 1 Number of FN3 FN3 Protein domains Angiopoietin 1 receptor 3 Contactin protein 4 Cytokine receptor common β chain 2 Down syndrome cell adhesion protein 6 Drosophila Sevenless protein 7 Erythropoietin receptor 1 Fibronectin 15 Growth hormone receptor 1 Insulin receptor 2 Insulin-like growth factor I receptor 3 Interferon-γ receptor β chain. 2 Interleukin-12 β chain 1 Interleukin-2 receptor β chain 1 Leptin receptor (LEP-R) 3 Leukemia inhibitory factor receptor (LIF-R) 6 Leukocyte common antigen 2 Neural cell adhesion protein L1 4 Prolactin receptor 2 Tenascin protein 15 Thrombopoietin receptor. 2 Tyrosine-protein kinase receptor Tie-1 3

Amino acid residues defining each loop and each beta-strand are shown for FN3 scaffold Tencon27 (SEQ ID NO: 27) in Table 2. Positions of each loop and beta-strand in tenascin C 3rd FN3 domain (TN3) (SEQ ID NO: 3) and Fibcon (SEQ ID NO: 58) are identical to that of Tencon27. Beta-strand residues can be identified using well known methods, for example, by analysis of 3-dimensional structures generated by x-ray diffraction, nuclear magnetic resonance, or molecular modeling. Where models are not available, analysis of sequence alignments with other known FN3 molecules can be used to predict the boundaries of strand and loop regions. Finally, computer algorithms can be used to predict the presence of beta strands from protein primary sequences.

TABLE 2 Tencon27 FN3 domain (SEQ ID NO: 27) A strand  1-12 AB loop 13-16 B strand 17-21 BC loop 22-28 C strand 29-37 CD loop 38-43 D strand 44-50 DE loop 51-54 E strand 55-59 EF loop 60-64 F strand 65-74 FG loop 75-81 G strand 82-89 Alternative Surfaces on FN3 Domains

The top (BC, DE, and FG) and the bottom (AB, CD, and EF) loops, e.g., the reported binding surfaces in the FN3 domains are separated by the beta-strands that form the center of the FN3 structure (FIGS. 1, 2A). Alternative surfaces residing on the two “sides” of the FN3 domains having different shapes than the surfaces formed by loops only can be visualized by rotating the FN3 domain structure by 90 degrees (FIGS. 2B, 2C). A slightly concave surface is formed at one side of the FN3 domain by two antiparallel beta-strands, the C and the F beta-strands, and the CD and FG loops, and is herein called the C-CD-F-FG surface. An alternative surface is also formed at the opposite side of the C-CD-F-FG surface by the A, B and E beta-strands and the AB and BC loops, herein called the A-AB-B-BC-E surface.

The alternative surfaces in the FN3 domains are encoded by non-contiguous stretches of amino acid in each FN3 domain. For example, Tencon27 C-CD-F-FG surface is formed by amino acid residues 29-43 and 65-81 of SEQ ID NO: 27, and the Tencon27 A-AB-B-BC-E surface formed by amino acid residues 1-28 and 55-59 of SEQ ID NO: 27, as shown in Table 2.

Protein Scaffolds Based on Randomizing Alternative Surfaces

One embodiment of the invention is an isolated protein scaffold comprising a fibronectin module of type III (FN3) domain comprising an alternative surface, wherein the alternative surface has at least one amino acid substitution in each beta-strand and each loop forming the alternative surface when compared to a reference FN3 domain.

In another embodiment, the protein scaffold of the invention specifically binds to a target molecule not specifically bound by the reference FN3 domain.

In another embodiment, the reference FN3 domain comprises a SEQ ID NO: 27.

In another embodiment, the protein scaffold of the invention comprises a C-CD-F-FG alternative surface formed by a C beta-strand, a CD loop, an F beta-strand, and a FG loop.

In another embodiment, the C beta-strand, the CD loop, the F beta-strand, or the FG loop forming the C-CD-F-FG alternative surface comprise certain amino acid sequences as shown in Tables 4 and 5 and in SEQ ID NOS: 45-48.

In another embodiment, the C beta-strand comprises an amino acid sequence DSFLIQYQE (SEQ ID NO: 33) having substitutions at 1, 2, 3, or 4 residues, the F beta-strand comprises an amino acid sequence TEYTVSIYGV (SEQ ID NO: 39) having substitutions at 1, 2, 3, 4, or 5 residues, the C beta-strand and the CD loop comprises an amino acid sequence DSFLIQYQESEKVGE (SEQ ID NO: 42) having substitutions at 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 residues, or the F beta-strand and the FG loop comprises and amino acid sequence TEYTVSIYGVKGGHRSN (SEQ ID NO: 43) having substitutions at 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11 residues.

In another embodiment, the C beta-strand and the F beta-strand comprise an amino acid sequence at least 67% identical to SEQ ID NO:33 and at least 70% identical to SEQ ID NO:39, respectively, the C beta-strand and the CD loop comprises an amino acid sequence at least 53% identical to SEQ ID NO: 42, or the F beta-strand and the FG loop comprises an amino acid sequence at least 65% identical to SEQ ID NO: 43.

In another embodiment, the protein scaffold of the invention comprises a FN3 domain comprising an amino acid sequence shown in SEQ ID NO: 28.

In another embodiment, the protein scaffold of the invention comprises a fibronectin module of type III (FN3) domain comprising:

-   -   an A beta-strand, an AB loop, a B beta-strand, a BC loop, a D         beta-strand, a DE loop, an E beta-strand, an EF loop and a G         beta-strand having amino acid sequences identical to SEQ ID NO:         27 at residues 1-12, 13-16, 17-21, 22-28, 44-50, 51-54, 55-59,         60-64, and 82-89, respectively;     -   a C beta-strand and a CD loop having an amino acid sequence at         least 53% identical to SEQ ID NO: 42; and     -   a F beta-strand and an FG loop having an amino acid sequence at         least 65% identical to SEQ ID NO: 43, optionally having at least         one substitution at amino acid positions corresponding to amino         acid residues 11, 14, 17, 37, 46, 73, or 86 of SEQ ID NO: 27,         wherein the protein scaffold specifically binds to a target         molecule not specifically bound by a reference FN3 domain.

In another embodiment, the protein scaffold of the invention comprises a FN3 domain comprising an amino acid sequence at least 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequences shown in SEQ ID NO: 27.

In another embodiment, the protein scaffold of the invention comprises an A-AB-B-BC-E alternative surface formed by an A beta-strand, an AB loop, a B beta-strand, a BC loop, and an E beta-strand.

In another embodiment, the A beta-strand, the AB loop, the B beta-strand, and the BC loop forming the A-AB-B-BC-E alternative surface comprise certain amino acid sequences as shown in Tables 4 and 5 and in SEQ ID NOS: 49-50.

In another embodiment, the A beta-strand, the AB loop, the B beta-strand and the BC loop comprise an amino acid sequence that is at least 59% identical to SEQ ID NO:44, and the E beta-strand comprises an amino acid sequence that is at least 60% identical to SEQ ID NO: 37.

In another embodiment, the protein scaffold of the invention comprises an FN3 domain comprising an amino acid sequence shown in SEQ ID NO: 61.

In another embodiment, an isolated protein scaffold of the invention comprises a fibronectin module of type III (FN3) domain comprising a fibronectin module of type III (FN3) domain comprising:

-   -   a C beta-strand, a CD loop, a D beta-strand, a DE loop, an EF         loop, an F beta-strand, an FG loop, and a G beta-strand having         amino acid sequences identical to SEQ ID NO: 27 at residues         29-37, 38-43, 44-50, 51-54, 60-64, 65-74, 75-81, and 82-89,         respectively;     -   an A beta-strand, an AB loop, a B beta-strand, and a BC loop         having an amino acid sequence that is at least 59% identical to         SEQ ID NO: 44; and     -   an E beta-strand having an amino acid sequence that is at least         60% identical to SEQ ID NO: 37, optionally having at least one         substitution at amino acid positions corresponding to amino acid         residues 11, 14, 17, 37, 46, 73, or 86 of SEQ ID NO: 27, wherein         the protein scaffold specifically binds to a target molecule not         specifically bound by a reference FN3 domain.

The FN3 domains specifically binding to a target molecule can be generated by randomizing a subset of the residues that form the alternative surface. For example, at least one, two, three, four, five, six, seven, eight, nine, or ten residues can be randomized in each beta-strand and each loop contributing to the alternative surface. Additional residues can be randomized to increase diversity of the library. For example, 20%, 30%, 40%, 50%, 60%, 70%, 75%, 80%, 85%, 90%, or 95% of the residues in each beta-strand and each loop forming the alternative surface may be randomized. Alternatively, FN3 domains specifically binding to a target molecule can be generated by randomizing a subset of the residues in the beta-strands contributing to the alternative surface, without randomizing any of the loops. For example, at least one, two, three, four, five, six, seven, eight, nine, or ten residues in each strand contributing to the alternative surface can be randomized. Library diversity can be increased by randomizing additional residues residing in the beta-strands. For example, 20%, 30%, 40%, 50%, 60%, 70%, 75%, 80%, 85%, 90%, or 95%, of the residues in each beta-strand forming the alternative surface may be randomized.

Beta-strands have a repeating structure with the side-chain of every other residue exposed to the surface of the protein. Surface exposed side-chains are determined by examination of three dimensional structures or by comparison to sequences of FN3 domains with known structure by multiple sequence alignment. All or a subset of surface exposed residues in the beta-strands contributing to the alternative surface may be chosen to be randomized. For example, Tencon27 (SEQ ID NO: 27) C-CD-F-FG alternative surface has four surface exposed residues in the C beta-strand (S30, L32, Q34, and Q36) and five surface exposed residues in the F beta-strand (E66, T68, S70, Y72, and V74), residue numbering based on SEQ ID NO: 27. One or more of these residues may be randomized to generate a library. Residues at the junction of the alternative surface, such as S30, E66 and V74 may or may not be randomized. Randomization of the buried residues of the beta-strands may result in the destabilization of the scaffold due to the loss of hydrophobic contacts in the core of the structure. The buried residues may be randomized so that only a subset of amino acids is used, for example only hydrophobic amino acids.

A subset or all residues in the loop regions contributing to the alternative surface may be randomized. For example, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or 13 positions may be substituted in the CD and/or FG loops contributing to the alternative surface. Glycine residues in the loops, such as G42, G76 and/or G77 in Tencon27, can provide flexibility and may or may not be randomized. Residues at the beta-strand/loop boundaries, such as E43 in Tencon27 may or may not be randomized. Additional residues in the beta-strand or loop regions may be included or excluded from randomization. For example, residues that appear to be required for stabilization identified based on, for example, analysis of crystal structures of the FN3 domains, may or may not be randomized. For example, S80 in Tencon27 makes contacts with the FN3 domain core to potentially stabilize the FG loop, and K75 partially faces away from the alternative surface. Thus, both these residues may be excluded from initial library design. In an exemplary FN3 domain library having randomized C-CD-F-FG surface, residues that can be randomized include residues at positions 30, 32, 34, 36, 38, 39, 40, 41, 42, 43, 66, 68, 70, 72, 74, 75, 76, 77, 78, 79, 80, or 81 of SEQ ID NO: 27. In an exemplary FN3 domain library having randomized A-AB-B-BC-E surface, residues that can be randomized include residues at positions 6, 8, 10, 11, 12, 13, 14, 15, 16, 18, 20, 22, 23, 24, 25, 26, 27, 55, and 57.

Diversity at loops contributing to alternative surfaces may be achieved by insertion and/or deletions of residues at loops. For example, the FG and/or CD loops may be extended by 1-22 amino acids, or decreased by 1-3 amino acids. The FG loop in Tencon27 is 7 amino acids long, whereas the corresponding loop in antibody heavy chains ranges from 4-28 residues. To provide maximum diversity, the loops contributing to alternative surfaces, for example, the FG loop, may be diversified in sequence as well as in length to correspond to the antibody CDR3 length range of 4-28 residues.

The resulting FN3 domains specifically binding to a target molecule can be further modified at residues residing outside of or within the alternative surface for the purpose of for example improving stability, reducing immunogenicity, enhancing binding affinity, on-rate, off-rate, half life, solubility, or any other suitable characteristics. In one way to achieve this goal, the scaffold proteins can be optionally prepared by a process of analysis of the parental sequences and various conceptual engineered products using three-dimensional models of the parental and engineered sequences. Three-dimensional models are commonly available and are familiar to those skilled in the art. Computer programs are available which illustrate and display probable three-dimensional conformational structures of selected candidate sequences and can measure possible immunogenicity (e.g., Immunofilter program of Xencor, Inc. of Monrovia, Calif.). Inspection of these displays permits analysis of the likely role of the residues in the functioning of the candidate sequence, for example, residues that influence stability of the scaffold protein or the ability of the candidate scaffold protein to bind its target molecule. In this way, residues can be selected and combined from the parent and reference sequences so that the desired characteristics, such as improved scaffold stability is achieved. Alternatively, or in addition to the above procedures, other suitable methods of engineering can be used as known in the art.

Desirable physical properties of FN3 domains of the invention include high thermal stability and reversibility of thermal folding and unfolding. Several methods have been applied to increase the apparent thermal stability of proteins and enzymes, including rational design based on comparison to highly similar thermostable sequences, design of stabilizing disulfide bridges, mutations to increase alpha-helix propensity, engineering of salt bridges, alteration of the surface charge of the protein, directed evolution, and composition of consensus sequences (Lehmann and Wyss, Curr Opin Biotechnol, 12, 371-375, 2001). High thermal stability may increase the yield of the expressed protein, improve solubility or activity, decrease immunogenicity, and minimize the need of a cold chain in manufacturing.

Residues that can be substituted to improve any characteristics of the FN3 domains of the invention can be determined by making the substitution and assaying for the desired characteristics of the scaffold. Exemplary FN3 domain-based scaffold with improved characteristics are the Tencon scaffold (SEQ ID NO: 16) or the Tencon27 scaffold (SEQ ID NO: 27) that is modified at one or more amino acid residue positions 11, 14, 17, 37, 46, 73, or 86.

In terms of loss of stability, i.e., “denaturing” or “denaturation” of a protein, is meant the process where some or all of the three-dimensional conformation imparting the functional properties of the protein has been lost with an attendant loss of activity and/or solubility. Forces disrupted during denaturation include intramolecular bonds, for example, electrostatic, hydrophobic, Van der Waals forces, hydrogen bonds, and disulfides. Protein denaturation can be caused by forces applied to the protein or a solution comprising the protein, such as mechanical force (for example, compressive or shear-force), thermal, osmotic stress, change in pH, electrical or magnetic fields, ionizing radiation, ultraviolet radiation and dehydration, and by chemical denaturants.

Measurement of protein stability and protein lability can be viewed as the same or different aspects of protein integrity. Proteins are sensitive or “labile” to denaturation caused by heat, by ultraviolet or ionizing radiation, changes in the ambient osmolarity and pH if in liquid solution, mechanical shear force imposed by small pore-size filtration, ultraviolet radiation, ionizing radiation, such as by gamma irradiation, chemical or heat dehydration, or any other action or force that may cause protein structure disruption. The stability of the molecule can be determined using standard methods. For example, the stability of a molecule can be determined by measuring the thermal melting (“TM”) temperature, the temperature in ° Celsius (° C.) at which ½ of the molecules become unfolded, using standard methods. Typically, the higher the TM, the more stable the molecule. In addition to heat, the chemical environment also changes the ability of the protein to maintain a particular three dimensional structure.

In one embodiment, the FN3 domains of the invention exhibit increased stability by at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95% or more compared to the same domain prior to engineering measured by the increase in the TM.

Chemical denaturation can likewise be measured by a variety of methods. Chemical denaturants include guanidinium hydrochloride, guanidinium thiocyanate, urea, acetone, organic solvents (DMF, benzene, acetonitrile), salts (ammonium sulfate lithium bromide, lithium chloride, sodium bromide, calcium chloride, sodium chloride); reducing agents (e.g. dithiothreitol, beta-mercaptoethanol, dinitrothiobenzene, and hydrides, such as sodium borohydride), non-ionic and ionic detergents, acids (e.g. hydrochloric acid (HCl), acetic acid (CH₃COOH), halogenated acetic acids), hydrophobic molecules (e.g. phosopholipids), and targeted denaturants. Quantitation of the extent of denaturation can rely on loss of a functional property, such as ability to bind a target molecule, or by physiochemical properties, such as tendency to aggregation, exposure of formerly solvent inaccessible residues, or disruption or formation of disulfide bonds.

In one embodiment, the scaffolds of the invention exhibit increased stability by at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95% or more compared to the same scaffold prior to engineering measured by using guanidinium hydrochloride as a chemical denaturant. Increased stability can be measured as a function of decreased tryptophan fluorescence upon treatment with increasing concentrations of guanidine hydrochloride using well known methods.

The FN3 domains specifically binding to a target molecule of the invention can be generated using any FN3 domain as a template for substitutions according to methods provided within. Exemplary FN3 domains having randomized alternative surfaces are the 3rd FN3 domain of tenascin C (TN3) (SEQ ID NO: 3), Tencon (SEQ ID NO: 16), Tencon27 (SEQ ID NO: 27), Fibcon (SEQ ID NO: 58), and the 10^(th) FN3 domain of fibronectin (FN10) (SEQ ID NO: 97). The amino acid positions delineating the alternative surfaces in Tencon27 are shown in Table 2 and FIG. 8, and are identical in Tencon, TN3, and Fibcon linear sequence. The amino acid positions delineating the alternative surface in FN10 is shown in FIG. 8. The residues forming the alternative surfaces in other FN3 domains can be identified by examination of three dimensional structures where available or by analysis of sequence alignments of FN3 domains by well known methods.

The FN3 domains of the invention may be generated as monomers, dimers, or multimers, for example, as a means to increase the valency and thus the avidity of target molecule binding, or to generate bi- or multispecific scaffolds simultaneously binding two or more different target molecules. The dimers and multimers may be generated by linking monospecific, bi- or multispecific protein scaffolds, for example, by the inclusion of an amino acid linker, for example a linker containing poly-glycine, glycine and serine, or alanine and proline. The use of naturally occurring as well as artificial peptide linkers to connect polypeptides into novel linked fusion polypeptides is well known in the literature (Hallewell et al., J Biol Chem 264, 5260-5268, 1989; Alfthan et al., Protein Eng. 8, 725-731, 1995; Robinson & Sauer, Biochemistry 35, 109-116, 1996; U.S. Pat. No. 5,856,456).

The FN3 domains of the present invention may be used as bispecific molecules wherein the first alternative surface in a domain has specificity for a first target molecule and the second alternative surface in the same domain has specificity for a second target molecule. An exemplary bispecific protein domain is a variant of Tencon27 which binds a first target molecule at the C-CD-F-FG surface, and a second target molecule at the A-AB-B-BC-E surface.

The FN3 domains of the present invention may incorporate other subunits for example via covalent interaction. All or a portion of an antibody constant region may be attached to the FN3 domain to impart antibody-like properties, especially those properties associated with the Fc region, e.g., complement activity, half-life, etc. For example, Fc effector functions such as Clq binding, complement dependent cytotoxicity (CDC), Fc receptor binding, antibody-dependent cell-mediated cytotoxicity (ADCC), phagocytosis, down regulation of cell surface receptors (e.g., B cell receptor; BCR), etc. can be provided and/or controlled by modifying residues in the Fc responsible for these activities (for review; see Strohl, Curr Opin Biotechnol. 20, 685-691, 2009).

Additional moieties may be incorporated into the FN3 domains of the invention such as toxin conjugates, albumin or albumin binders, polyethylene glycol (PEG) molecules, such as PEG5000 or PEG20,000, fatty acids and fatty acid esters of different chain lengths, for example laurate, myristate, stearate, arachidate, behenate, oleate, arachidonate, octanedioic acid, tetradecanedioic acid, octadecanedioic acid, docosanedioic acid, and the like, polylysine, octane, carbohydrates (dextran, cellulose, oligo- or polysaccharides) for desired properties. These moieties may be direct fusions with the protein scaffold coding sequences and may be generated by standard cloning and expression techniques. Alternatively, well known chemical coupling methods may be used to attach the moieties to recombinantly produced FN3 domains of the invention.

FN3 domains incorporating additional moieties may be compared for functionality by several well known assays. For example, altered FN3 domain properties due to incorporation of Fc domains and/or Fc domain variants may be assayed in Fc receptor binding assays using soluble forms of the receptors, such as the FcγRI, FcγRII, FcγRIII or FcRn receptors, or using well known cell-based assays measuring for example ADCC or CDC, or evaluating protein scaffold pharmacokinetic properties in in vivo models.

Generation and Production of FN3 Domain Proteins

One embodiment of the invention is a method of making a library of fibronectin module of type III (FN3) domains comprising an alternative surface, wherein the alternative surface has at least one amino acid substitution when compared to a reference FN3 domain, comprising: providing a polynucleotide encoding a reference FN3 domain; generating a library of polynucleotide sequences of the reference FN3 domain by randomizing the alternative surface; translating the library in vitro or expressing the library in a host.

Another embodiment of the invention is a method of making a library of fibronectin module of type III (FN3) domains having a diversified C-CD-F-FG alternative surface formed by a C beta-strand, a CD loop, an F beta-strand, and an FG loop, comprising providing a reference FN3 domain polypeptide having the amino acid sequence at least 80% identical to that of SEQ ID NO: 27; introducing diversity into the reference FN3 domain polypeptide by mutating at least one C beta-strand residue and at least one F beta-strand residue to form the FN3 domain library having the diversified C-CD-F-FG alternative surface.

In the methods of making the library of the invention, 1, 2, 3 or 4 residues in the C beta-strand can be mutated with the proviso that S30 is not mutated (residue numbering according to SEQ ID NO: 27).

In the methods of making the library of the invention, the C beta-strand residues L32, Q34 and Q36 can be mutated (residue numbering according to SEQ ID NO: 27).

In the methods of making the library of the invention, 1, 2, 3 or 4 residues in the F beta-strand can be mutated with the proviso that E66 is not mutated (residue numbering according to SEQ ID NO: 27).

In the methods of making the library of the invention, the F-beta strand residues T68, S70 and Y72 can be mutated (residue numbering according to SEQ ID NO: 27).

In the methods of making the library of the invention, 1, 2, 3 or 4 residues in the CD loop residues can be mutated with the proviso that G42 and E43 are not mutated (residues numbering according to SEQ ID NO: 27).

In the methods of making the library of the invention, the residues S38, E39, K40 and V41 in the CD loop can be mutated.

In the methods of making the library of the invention, 1, 2, 3 or 4 residues in the FG loop can be mutated with the proviso that the residues K75, G76, G77 and S80 are not mutated (residue numbering according to SEQ ID NO: 27).

In the methods of making the library of the invention, the residues H78, R79 and N81 in the FG loop can be mutated (residue numbering according to SEQ ID NO: 27).

In the methods of making the library of the invention, the reference FN3 domain comprises an amino acid sequence of SEQ ID NO: 27, optionally comprising at least one substitution at amino acid positions 11, 14, 17, 37, 46, 73, or 86.

Other reference FN3 domains may be used in the methods of the invention, such as Tencon (SEQ ID NO: 16) or variants thereof as shown in SEQ ID NOS: 17-26 and in Table 3.

Another embodiment of the invention is a library produced by the methods of the invention.

Generation of the scaffold proteins, FN3 domains (or modules) of the invention, is typically achieved at the nucleic acid level. The libraries of the FN3 domains of the invention having substituted codons at one or more specific residues can be synthesized for example using standard PCR cloning methods, or chemical gene synthesis according to methods described in U.S. Pat. Nos. 6,521,427 and 6,670,127. Codons can be randomized using well known methods, for example degenerate oligonucleotides matching the designed diversity, or using Kunkel mutagenesis Kunkel et al., Methods Enzymol. 154, 367-382, 1987).

Libraries can be randomized at chosen codons using a random or defined set of amino acids. For example, variants in the library having random substitutions can be generated using NNK codons, which encode all 20 naturally occurring amino acids. In other diversification schemes, DVK codons can be used to encode amino acids Ala, Trp, Tyr, Lys, Thr, Asn, Lys, Ser, Arg, Asp, Glu, Gly, and Cys. Alternatively, NNS codons can be used to give rise to all 20 amino acid residues and simultaneously reducing the frequency of stop codons. The codon designations are according to the well known IUB code.

The FN3 domains of the invention as any other proteins are prone to a variety of physical and/or chemical instabilities, resulting in adverse effects on the downstream processing. For instance, physical and chemical instability may lead to aggregation, degradation, reduced product yield, loss of potency, increased potential for immunogenicity, molecular heterogeneity, and loss of activity. Thus, presence of possible instability-inducing residues and recognition sequences may be minimize during the design of the libraries. For example, surface exposed methionine and tryptophan may be oxidized in storage conditions, possibly leading to loss in the protein scaffold potency. Presence of asparagine, in addition to contributing to well known N-glycosylation recognition sites (NXS/T) may be deamidated when followed by glycine, possibly generating heterogeneicity (Robinson, Proc Natl Acad Sci USA, 99, 5283-5288, 2002). Some or all of these amino acids thus may or may not be omitted from the mix used to randomize selected position. Furthermore, cysteine and proline may be omitted to minimize disulphide bridge formation and disruption of beta sheets.

Libraries of FN3 domains with biased amino acid distribution at positions to be diversified can be synthesized for example using Slonomics® technology (http:_//www_sloning_com). This technology uses a library of pre-made double stranded triplets that act as universal building blocks sufficient for thousands of gene synthesis processes. The triplet library represents all possible sequence combinations necessary to build any desired DNA molecule.

Synthesis of oligonucleotides with selected nucleotide “degeneracy” at certain positions is well known in that art, for example the TRIM approach (Knappek et al., J Mol Biol 296, 57-86, 1999; Garrard & Henner, Gene 128,103-109, 1993). Such sets of nucleotides having certain codon sets can be synthesized using commercially available nucleotide or nucleoside reagents and apparatus.

In an exemplary diversification scheme, Tencon27 FN3 domain (SEQ ID NO: 27) residues L32, Q34 and Q36 in the C beta-strand, S38, E39, K40 and V41 in the CD loop, T68, S70 and Y72 in the F beta-strand, and H78, R79, and N81 in the FG loop are randomized with NNS codons.

Standard cloning and expression techniques are used to clone the libraries into a vector or synthesize double stranded cDNA cassettes of the library, to express, or to translate the libraries in vitro. For example, cis-display can be used to ligate DNA fragments encoding the scaffold proteins to a DNA fragment encoding RepA to generate a pool of protein-DNA complexes formed after in vitro translation wherein each protein is stably associated with the DNA that encodes it (U.S. Pat. No. 7,842,476; Odegrip et al., Proc Natl Acad Sci USA 101, 2806-2810, 2004). Other methods can be used, for example ribosome display (Hanes and Pluckthun, Proc Natl Acad Sci USA, 94, 4937-4942, 1997), mRNA display (Roberts and Szostak, Proc Natl Acad Sci USA, 94, 12297-12302, 1997), or other cell-free systems (U.S. Pat. No. 5,643,768). The libraries of protein scaffolds may be expressed as fusion proteins displayed on the surface for example of any suitable bacteriophage. Methods for displaying fusion polypeptides on the surface of a bacteriophage are well known (U.S. Pat. Pub. No. 2011/0118144; Int. Pat. Pub. No. WO2009/085462; U.S. Pat. Nos. 6,969,108; 6,172,197; 5,223,409; 6,582,915; 6,472,147).

Screening

Screening engineered protein FN3 domains or libraries of FN3 domain variants for specific binding to target molecules can be achieved for example by producing the library using cis display as described in Examples and in Odegrip et al., Proc Natl Acad Sci USA 101, 2806-2810, 2004, and assaying the library for specific binding to a target molecule by any method known in the art. Exemplary well known methods which can be used are ELISA, sandwich immunoassays, and competitive and non-competitive assays (see, e.g., Ausubel et al., eds, 1994, Current Protocols in Molecular Biology, Vol. 1, John Wiley & Sons, Inc., New York).

The FN3 domains of the invention can bind human or other mammalian proteins with a wide range of affinities (K_(D)). Typically a FN3 domain of the present invention can bind to a target protein with a K_(D) equal to or less than about 10⁻⁷ M, 10⁻⁸ M, 10⁻⁹ M, 10⁻¹⁰ M, 10⁻¹¹ M, 10⁻¹² M, 10⁻¹³ M, 10⁻¹⁴ M, or 10⁻¹⁵ M as determined by surface plasmon resonance or the Kinexa method, as practiced by those of skill in the art. The affinity of a FN3 domain for an antigen can be determined experimentally using any suitable method. (See, for example, Berzofsky, et al., “Antibody-Antigen Interactions,” In Fundamental Immunology, Paul, W. E., Ed., Raven Press: New York, N.Y. (1984); Kuby, Janis Immunology, W. H. Freeman and Company: New York, N.Y. (1992); and methods described herein). The measured affinity of a particular FN3 domain-antigen interaction can vary if measured under different conditions (e.g., osmolarity, pH). Thus, measurements of affinity and other antigen-binding parameters (e.g., K_(D), K_(on), K_(off)) are preferably made with standardized solutions of protein scaffold and antigen, and a standardized buffer, such as the buffer described herein.

Nucleic Acid Molecules and Vectors

The invention provides for nucleic acids encoding the FN3 domains of the invention as isolated polynucleotides or as portions of expression vectors or as portions of linear DNA sequences, including linear DNA sequences used for in vitro transcription/translation, vectors compatible with prokaryotic, eukaryotic or filamentous phage expression, secretion and/or display of the compositions or directed mutagens thereof. Certain exemplary polynucleotides are disclosed herein, however, other polynucleotides which, given the degeneracy of the genetic code or codon preferences in a given expression system, encode the protein scaffolds and libraries of the protein scaffolds of the invention are also within the scope of the invention.

The polynucleotides of the invention may be produced by chemical synthesis such as solid phase polynucleotide synthesis on an automated polynucleotide synthesizer and assembled into complete single or double stranded molecules. Alternatively, the polynucleotides of the invention may be produced by other techniques such a PCR followed by routine cloning. Techniques for producing or obtaining polynucleotides of a given known sequence are well known in the art.

The polynucleotides of the invention may comprise at least one non-coding sequence, such as a promoter or enhancer sequence, intron, polyadenylation signal, a cis sequence facilitating RepA binding, and the like. The polynucleotide sequences may also comprise additional sequences encoding additional amino acids that encode for example a marker or a tag sequence such as a histidine tag or an HA tag to facilitate purification or detection of the protein, a signal sequence, a fusion protein partner such as RepA, Fc or bacteriophage coat protein such as pIX or pIII.

An exemplary polynucleotide comprises sequences for a Tac promoter, sequences encoding the FN3 domain library and repA, cis element, and a bacterial origin of replication (ori). Another exemplary polynucleotide comprises a pelB or ompA signal sequence, pIII or pIX bacteriophage coat protein, FN3 domain, and a polyA site. Exemplary polynucleotides encoding the TCL14 library and Tencon27 are shown in SEQ ID NOs: 100 and 101, respectively.

Another embodiment of the invention is a vector comprising at least one polynucleotide of the invention. Such vectors may be plasmid vectors, viral vectors, vectors for baculovirus expression, transposon based vectors or any other vector suitable for introduction of the polynucleotides of the invention into a given organism or genetic background by any means. Such vectors may be expression vectors comprising nucleic acid sequence elements that can control, regulate, cause or permit expression of a polypeptide encoded by such a vector. Such elements may comprise transcriptional enhancer binding sites, RNA polymerase initiation sites, ribosome binding sites, and other sites that facilitate the expression of encoded polypeptides in a given expression system. Such expression systems may be cell-based, or cell-free systems well known in the art.

Host Cell Selection or Host Cell Engineering

A FN3 domain of the present invention can be optionally produced by a cell line, a mixed cell line, an immortalized cell or clonal population of immortalized cells, as well known in the art. See, e.g., Ausubel, et al., ed., Current Protocols in Molecular Biology, John Wiley & Sons, Inc., NY, N.Y. (1987-2001); Sambrook, et al., Molecular Cloning: A Laboratory Manual, 2^(nd) Edition, Cold Spring Harbor, N.Y. (1989); Harlow and Lane, Antibodies, a Laboratory Manual, Cold Spring Harbor, N.Y. (1989); Colligan, et al., eds., Current Protocols in Immunology, John Wiley & Sons, Inc., NY (1994-2001); Colligan et al., Current Protocols in Protein Science, John Wiley & Sons, NY, N.Y., (1997-2001).

The host cell chosen for expression may be of mammalian origin or may be selected from COS-1, COS-7, HEK293, BHK21, CHO, BSC-1, Hep G2, 653, SP2/0, 293, HeLa, myeloma, lymphoma, yeast, insect or plant cells, or any derivative, immortalized or transformed cell thereof. Alternatively, the host cell may be selected from a species or organism incapable of glycosylating polypeptides, e.g. a prokaryotic cell or organism, such as BL21, BL21(DE3), BL21-GOLD(DE3), XL1-Blue, JM109, HMS174, HMS174(DE3), and any of the natural or engineered E. coli spp, Klebsiella spp., or Pseudomonas spp strains.

Uses of FN3 Domains of the Invention

The compositions of the FN3 domain (module)-based molecules described herein and generated by any of the above described methods may be used to diagnose, monitor, modulate, treat, alleviate, help prevent the incidence of, or reduce the symptoms of human disease or specific pathologies in cells, tissues, organs, fluid, or, generally, a host. A FN3 domain engineered for a specific purpose may be used to treat an immune-mediated or immune-deficiency disease, a metabolic disease, a cardiovascular disorder or disease; a malignant disease; a neurologic disorder or disease; an infection such as a bacterial, viral or parasitic infection; or other known or specified related condition including swelling, pain, and tissue necrosis or fibrosis.

Such a method can comprise administering an effective amount of a composition or a pharmaceutical composition comprising at least one FN3 domain specifically binding a target molecule to a cell, tissue, organ, animal or patient in need of such modulation, treatment, alleviation, prevention, or reduction in symptoms, effects or mechanisms. The effective amount can comprise an amount of about 0.001 to 500 mg/kg per single (e.g., bolus), multiple or continuous administration, or to achieve a serum concentration of 0.01-5000 μg/ml serum concentration per single, multiple, or continuous administration, or any effective range or value therein, as done and determined using known methods, as described herein or known in the relevant arts.

Pharmaceutical Compositions Comprising FN3 Domain-Based Proteins

The FN3 domains specifically binding target molecules which are modified or unmodified, monomers, dimers, or multimers, mono-, bi- or multi-specific, can be isolated using separation procedures well known in the art for capture, immobilization, partitioning, or sedimentation, and purified to the extent necessary for commercial applicability.

For therapeutic use, the FN3 domains specifically binding a target molecule may be prepared as pharmaceutical compositions containing an effective amount of the FN3 domain as an active ingredient in a pharmaceutically acceptable carrier. The term “carrier” refers to a diluent, adjuvant, excipient, or vehicle with which the active compound is administered. Such vehicles can be liquids, such as water and oils, including those of petroleum, animal, vegetable or synthetic origin, such as peanut oil, soybean oil, mineral oil, sesame oil and the like. For example, 0.4% saline and 0.3% glycine can be used. These solutions are sterile and generally free of particulate matter. They may be sterilized by conventional, well-known sterilization techniques (e.g., filtration). The compositions may contain pharmaceutically acceptable auxiliary substances as required to approximate physiological conditions such as pH adjusting and buffering agents, stabilizing, thickening, lubricating and coloring agents, etc. The concentration of the agent of the invention in such pharmaceutical formulation can vary widely, i.e., from less than about 0.5%, usually at or at least about 1% to as much as 15 or 20% by weight and will be selected primarily based on required dose, fluid volumes, viscosities, etc., according to the particular mode of administration selected. Suitable vehicles and formulations, inclusive of other human proteins, e.g., human serum albumin, are described, for example, in e.g. Remington: The Science and Practice of Pharmacy, 21^(st) Edition, Troy, D. B. ed., Lipincott Williams and Wilkins, Philadelphia, Pa. 2006, Part 5, Pharmaceutical Manufacturing pp 691-1092, See especially pp. 958-989.

The mode of administration for therapeutic use of the FN3 domains specifically binding a target molecule may be any suitable route that delivers the agent to the host, such as parenteral administration, e.g., intradermal, intramuscular, intraperitoneal, intravenous or subcutaneous, pulmonary; transmucosal (oral, intranasal, intravaginal, rectal); using a formulation in a tablet, capsule, solution, powder, gel, particle; and contained in a syringe, an implanted device, osmotic pump, cartridge, micropump; or other means appreciated by the skilled artisan, as well known in the art. Site specific administration may be achieved by for example intrarticular, intrabronchial, intraabdominal, intracapsular, intracartilaginous, intracavitary, intracelial, intracerebellar, intracerebroventricular, intracolic, intracervical, intragastric, intrahepatic, intracardial, intraosteal, intrapelvic, intrapericardiac, intraperitoneal, intrapleural, intraprostatic, intrapulmonary, intrarectal, intrarenal, intraretinal, intraspinal, intrasynovial, intrathoracic, intrauterine, intravascular, intravesical, intralesional, vaginal, rectal, buccal, sublingual, intranasal, or transdermal delivery.

While having described the invention in general terms, the embodiments of the invention will be further disclosed in the following examples that should not be construed as limiting the scope of the claims.

EXAMPLE 1 Tencon Scaffold

Tencon Design

The third fibronectin module of type III (Fn3) domain from human tenascin C (SEQ ID NO: 3) can be used as a protein scaffold that can be engineered to bind to specific target molecules. The melting temperature of this domain is 54° C. in PBS in its native form.

In order to produce a protein scaffold with a similar structure and improved physical properties, such as an improved thermal stability, a consensus sequence was designed based on an alignment of 15 FN3 domains from human tenascin C (shown in SEQ ID NOS: 1-15). The 15 selected FN3 domains have sequence identities to each other ranging from 13 to 80%, with an average sequence identity among pairs of 29%. A consensus sequence designated as Tencon (SEQ ID NO: 16) was designed by incorporating the most conserved (frequent) amino acid at each position (see U.S. Pat. Pub. No. 2010/0216708). In pairwise alignments, Tencon is identical to the FN3 domains from tenascin C at 34-59% of positions with an average sequence identity of 43%.

Tencon Expression and Purification

The amino acid sequence of Tencon was back translated, resulting in the cDNA sequence shown in SEQ ID NO: 59. The cDNA was amplified and cloned into modified pET15 vector using routine methods. The protein was expressed as a C-terminal His₆-fusion protein in soluble form in E. coli, and purified using standard Ni-NTA agarose using elution in 500 mM imidazole. The desired fractions were pooled and dialyzed into PBS pH 7.4. As a second purification step the protein was loaded onto a Superdex-75 HiLoad 16/60 column (GE Healthcare) equilibrated in PBS. The fractions containing Tencon were pooled and concentrated using a Centriprep UltraCel YM-3 concentrator (Amicon). SDS-PAGE analysis showed that Tencon migrates between 6 and 14 kDa, in agreement with the expected mass of 10.7 kDa for the monomeric protein. A yield of >50 mg of pure Tencon protein per liter of culture was obtained.

Tencon Biophysical Characterization

The structure and stability of Tencon was characterized by circular dichroism spectroscopy (CD) and differential scanning calorimetry (DSC). CD measurements were made on an AVIV spectrometer at 20° C. in PBS and a concentration of 0.2 mg/mL. The spectrum showed a minimum at 218 nm, suggestive of beta-sheet structure as expected for a protein belonging to the FN3 family. DSC data was obtained by heating 0.5 mg/mL solutions of the 3^(rd) FN3 domain from tenascin C (TN3) or Tencon in PBS from 35° C. to 95° C. at a rate of 1° C./minute in an N-DSCII calorimeter (Applied Thermodynamics). From this data, melting temperatures of 54° C. and 78° C. were calculated for TN3 and Tencon, respectively, using CpCalc (Applied Thermodynamics) software. The folding and unfolding of both domains is reversible at these temperatures. Thus, the generated Tencon scaffold demonstrates an improved thermal stability when compared to that of the TN3. Based on this stability increase, the Tencon scaffold is likely to be more amenable to amino acid substitution and easier to manufacture. Mutations that decrease protein stability are likely to be better tolerated in the context of a more stable scaffold and thus a scaffold with enhanced stability is likely to yield more functional, well folded binders from a library of scaffold variants.

Tencon Display on M13 Phage

The cDNA (SEQ ID NO: 59) encoding the Tencon amino acid sequence was subcloned into the phagemid expression vector pPep9 (Int. Pat. Pub. No. WO2008/079973) by standard PCR and restriction digest cloning, resulting in the vector pTencon-pIX. This vector expresses N-terminally Myc-tagged Tencon as a C-terminal fusion to the N-terminus of the bacteriophage M13 pIX protein under Lac promoter (allowing for lower levels of expression without IPTG and increased expression after the addition of IPTG) utilizing the OmpA signal sequence. A short TSGGGGS linker (SEQ ID NO: 60) was inserted between Tencon and pIX to prevent steric interactions between these proteins.

For confirmation of display on the surface of the M13 phage particle, single colony transformants of pTencon-pIX in XL1-Blue E. coli were grown at 37° C. until reaching mid-log phase and rescued with 6¹⁰ pfu of VCSM13 helper phage. Supernatants were collected from the rescued cultures after 16 hour expansion in 2YT media supplemented with ampicillin followed by 1 mM IPTG induction, centrifuged at 4000×g for 20 minutes and stored at 4° C. for analysis.

Binding of the phage particles to an anti-Myc antibody (Life Technologies, Carlsbad, Calif.) was used to confirm the display of the Myc-Tencon construct on the M13 phage surface. A Maxisorp plate was coated overnight at a concentration of 2.5 μg/mL with anti-Myc or an anti-av antibody (negative control) and blocked with SuperBlock T20 (Thermo Scientific, Rockford Ill.). Two-fold serial dilutions of the phagemid culture supernatant described above were made in PBS and added to the wells of the coated plate. After 1 hour, the plate was washed with TBST and an anti-M13 HRP antibody was added to each well and washed with TBST following a 1-hour incubation. The Roche BD ELISA POD substrate was added and luminescence detected on a Tecan plate reader

EXAMPLE 2 Stabilizing Mutations in Tencon

Tencon libraries, FG7 and BC6/FG7, designed to introduce diversity into the FG and FG and BC loops simultaneously have been described (U.S. Pat. Pub. No. 2010/0255056; U.S. Pat. Pub. No. 2010/0216708).

Design of Variants

Mutants were designed to improve the folding stability of Tencon (SEQ ID NO: 16). Several point mutations were made to produce substitution of individual residues of SEQ ID NO: 16, such as N46V (Tencon17; SEQ ID NO:17), E14P (Tencon18; SEQ ID NO:18), E11N (Tencon19; SEQ ID NO:19), E37P (Tencon20; SEQ ID NO:20), and G73Y (Tencon21; SEQ ID NO:21) which were predicted to improve the scaffold stability by the program PoPMuSiC v2.0 (Dehouck et al., Bioinformatics, 25, 2537-2543, 2009). The mutant E86I (Tencon22; SEQ ID NO:22) had been previously found to stabilize a homologous protein, the 3^(rd) FN3 domain from human tenascin C (WO2009/086116). The L17A mutation (Tencon26; SEQ ID NO: 26) was found to significantly stabilize Tencon during alanine scanning experiments in which all loop residues of Tencon were replaced with alanine independently (data not shown). Following an initial round of stability assays, the combinatorial mutants N46V/E86I (Tencon23; SEQ ID NO:23), E14P/N46V/E86I (Tencon24; SEQ ID NO:24), and L17A/N46V/E86I (Tencon25; SEQ ID NO:25) were produced to further increase stability.

Expression and Purification

Mutations in the Tencon coding sequence were made using a QuikChange mutagenesis kit (Stratagene), and the mutant proteins were expressed and purified using standard protocols as HIS₆ fusion proteins. The proteins were eluted from Ni-NTA (Novagen) columns in 50 mM sodium phosphate pH 7.4, 500 mM NaCl, and 250 mM imidazole. After elution, the proteins were dialyzed into PBS pH 7.4.

Characterization of Thermal Stability

The thermal stabilities of Tencon and each mutant protein in pBS pH 7.4 (2-3 mg/mL) were measured by capillary differential scanning calorimetry (DSC). Melting temperatures were measured for these samples using a VP-DSC instrument equipped with an autosampler (MicroCal, LLC). Samples were heated from 10° C. to 95° C. or 100° C. at a rate of 1° C. per minute. A buffer only scan was completed between each sample scan in order to calculate a baseline for integration. Data were fit to a two state unfolding model following subtraction of the buffer only signal. Reversibility of thermal denaturation was determined by repeating the scan for each sample without removing it from the cell. Reversibility was calculated by comparing the area under the curve from the 1^(st) scan with the 2^(nd) scan. Results of the DSC experiments are presented in Table 3 as the values derived from complete melting curves (Tm(Kcal)). Single mutants Tencon17, Tencon18, Tencon19, and Tencon22 had improved thermal stability compared to the parent Tencon sequence. Only Tencon21 was significantly destabilizing. Combinatorial mutants Tencon23, Tencon24, and Tencon25 and all had a significantly larger enhancement of the stability, indicating that the designed mutations are additive with respect to improving thermal stability.

Denaturation by Guanidine Hydrochloride

The abilities of Tencon and each mutant to remain folded upon treatment with increasing concentrations of guanidine hydrochloride (GdmCl) as measured by tryptophan fluorescence were used to assess stability. Tencon contains only one tryptophan residue. The tryptophan residue is buried within the hydrophobic core and thus fluorescence emission at 360 nm is a sensitive measure of the folded state of this protein. 200 μL of a solution containing 50 mM sodium phosphate pH 7.0, 150 mM NaCl, and variable concentrations of GdmCl from 0.48 to 6.63 M were pipetted into black, non-binding, 96-well plates (Greiner) in order to produce a 17 point titration. 10 μL of a solution containing the Tencon mutants were added to each well across the plate to make a final protein concentration of 23 μM and mixed by pipetting up and down gently. After incubation at room temperature for 24 hours, fluorescence was read using a Spectramax M5 plate reader (Molecular Devices, Sunnyvale, Calif.) with excitation at 280 nm and emission at 360 nm. Fluorescence signal was converted to fraction unfolded using the equation (Pace, Methods Enzymol 131:266-280, 1986): f _(u)=(y _(F) −y)/(y _(F) −y _(u)) Where y_(F) is the fluorescence signal of the folded sample and y_(u) of the unfolded sample. The mid-points of the unfolding transition and slope of the transition were determined by fitting to the equation below (Clarke et al., 1997):

$F = \frac{\left( {\alpha_{N} + {\beta_{N}\lbrack D\rbrack}} \right) + {\left( {\alpha_{D} + {\beta_{D}\lbrack D\rbrack}} \right){\exp\left( {{m\left( {\lbrack D\rbrack - \lbrack D\rbrack_{50\%}} \right)}\text{/}{RT}} \right)}}}{1 + {\exp\left( {{m\left( {\lbrack D\rbrack - \lbrack D\rbrack_{50\%}} \right)}\text{/}{RT}} \right)}}$

Where F is the fluorescence at the given denaturant concentration, α_(N) and α_(D) are the y-intercepts of the native and denatured state, β_(X) and β_(D) are the slopes of the baselines for the native and denatured state, [D] is the concentration of GdmCl, [D]_(30%) the GdmCl concentration at which point 50% of the sample is denatured, m the slope of the transition, R the gas constant, and T the temperature. The free energy of folding for each sample was estimated using the equation (Pace 1986 supra; Clarke et al., J Mol Biol 270, 771-778, 1997): ΔG=m[D]_(30%).

It is often difficult to accurately measure the slope of the transition, m, for such curves. Additionally, the mutations described here are not expected to alter the folding mechanism of tencon. Thus, the m value for each mutant was measured and the values averaged (Pace 1986 supra) to produce an m=3544 cal/mol/M used for all free energy calculations. The results of these calculations are presented in Table 3. The results for GdmCl unfolding experiments demonstrate that the same mutants that stabilize Tencon with respect to thermal stability also stabilize the protein against GdmCl induced denaturation.

TABLE 3 Tm [D]_(50%) DG(H₂O) SEQ ID Construct Mutations (Kcal) (M) (kcal/mol) NO: Tencon 78.04 3.4 12 16 Tencon17 N46V 81.88 3.6 12.8 17 Tencon18 E14P 82.77 3.5 12.4 18 Tencon19 E11N 79 3.4 12 19 Tencon20 E37P 77.4 3.4 12 20 Tencon21 G73Y 67.56 2.4 8.5 21 Tencon22 E86I 82.78 3.7 13.1 22 Tencon23 N46V/E86I 86.65 4.1 14.5 23 Tencon24 E14P/N46V/E86I 87.47 4 14.2 24 Tencon25 L17A/N46V/E86I 92.73 5.1 18.1 25 Tencon26 L17A 84.9 4.6 16.2 26 Size Exclusion Chromatography

Size exclusion chromatography (SEC) was used to assess the aggregation state of Tencon and each Tencon variant. 5 μL of each sample were injected onto a Superdex 75 5/150 column (GE Healthcare) at a flow rate of 0.3 mL/min with a PBS mobile phase. Elution from the column was monitored by absorbance at 280 nm. In order to assess the aggregation state, the column was previously calibrated with globular molecular weight standards (Sigma). All of the samples tested, with the exception of Tencon21, eluted in one peak at an elution volume consistent with that of a monomeric sample. Tencon21 eluted with 2 peaks, indicating the presence of aggregates.

EXAMPLE 3 Generation of Tencon Libraries Having Alternative Binding Surfaces Design of the TCL14 Library

The choice of residues to be randomized in a particular library design governs the overall shape of the interaction surface created. X-ray crystallographic analysis of an FN3 domain containing scaffold protein selected to bind maltose binding protein (MBP) from a library in which the BC, DE, and FG loops were randomized was shown to have a largely curved interface that fits into the active site of MBP (Koide et al., Proc Natl Acad Sci USA, 104, 6632-6637, 2007). In contrast, an ankyrin repeat scaffold protein that was selected to bind to MBP was found to have a much more planar interaction surface and to bind to the outer surface of MBP distant from the active site (Binz et al., Nat Biotechnol, 22, 575-58, 2004). These results suggest that the shape of the binding surface of a scaffold molecule (curved vs. flat) may dictate what target proteins or specific epitopes on those target proteins are able to be bound effectively by the scaffold. Published efforts around engineering protein scaffolds containing FN3 domains for protein binding has relied on engineering adjacent loops (FIG. 1) for target binding, thus producing curved binding surfaces. This approach may limit the number of targets and epitopes accessible by such scaffolds.

Tencon and other FN3 domains contain two sets of CDR-like loops lying on the opposite faces of the molecule, the first set formed by the BC, DE, and FG loops, and the second set formed by the AB, CD, and EF loops. The two sets of loops are separated by the beta-strands that form the center of the FN3 structure (FIGS. 1, 2A). If the image of the Tencon structure presented in FIG. 1 is rotated by 90 degrees, an alternative surface can be visualized (FIG. 2B). This slightly concave surface is formed by the CD and FG loops and two antiparallel beta-strands, the C and the F beta-strands, and is herein called the C-CD-F-FG surface (FIG. 2B). The C-CD-F-FG surface can be used as a template to design libraries of protein scaffold interaction surfaces by randomizing a subset of residues that form the surface. Beta-strands have a repeating structure with the side chain of every other residue exposed to the surface of the protein. Thus, a library can be made by randomizing some or all surface exposed residues in the beta strands. By choosing the appropriate residues in the beta-strands, the inherent stability of the Tencon scaffold should be minimally compromised while providing a unique scaffold surface for interaction with other proteins.

A new library, herein called TCL14 (SEQ ID NO: 28), was designed into Tencon25 scaffold (SEQ ID NO: 25) having an additional E11R substitution (Tencon27, SEQ ID NO: 27) (FIGS. 2B, 3). Positions of the loops and strands and their sequences are shown in Table 4 and Table 5 for Tencon27 (SEQ ID NO: 27) and TCL14 (SEQ ID NO: 28), respectively. In Table 5, “X” indicates any amino acid.

Tencon27 (SEQ ID NO: 27): LPAPKNLVVSRVTEDSARLSWTAPDAAFDSFLIQYQESEKVGEAIVLTVP GSERSYDLTGLKPGTEYTVSIYGVKGGHRSNPLSAIFTT TCL14 library (SEQ ID NO: 28): LPAPKNLVVSRVTEDSARLSWTAPDAAFDSFXIXYXEXXXXGEAIVLTVP GSERSYDLTGLKPGTEYXVXIXGVKGGXXSXPLSAIFTT; wherein “X” is any amino acid.

The two beta strands forming the C-CD-F-FG surface in Tencon27 have a total of 9 surface exposed residues that could be randomized; C-strand: S30, L32, Q34, Q36; F-strand: E66, T68, S70, Y72, and V74, while the CD loop has 6 potential residues: S38, E39, K40, V41, G42, and E43 and the FG loop has 7 potential residues: K75, G76, G77, H78, R79, S80, and N81 (FIG. 5). Select residues were chosen for inclusion in the TCL14 design due to the larger theoretical size of the library if all 22 residues were randomized.

Thirteen positions in Tencon27 (SEQ ID NO: 27) were chosen for randomizing: L32, Q34 and Q36 in C-strand, S38, E39, K40 and V41 in CD-loop, T68, S70 and Y72 in F-strand, H78, R79, and N81 in FG-loop. In the C and F strands S30 and E66 were not randomized as they lie just beyond the CD and FG loops and do not appear to be as apparently a part of the C-CD-F-FG surface. For the CD loop, G42 and E43 were not randomized as glycine, providing flexibility, can be valuable in loop regions, and E43 lies at the junction of the surface. The FG loop had K75, G76, G77, and S80 excluded. The glycines were excluded for the reasons above while careful inspection of the crystal structures revealed S80 making key contacts with the core to help form the stable FG loop. K75 faces away from the surface of the C-CD-F-FG surface and was a less appealing candidate for randomization. Although the above mentioned residues were not randomized in the original TCL14 design, they could be included in subsequent library designs to provide additional diversity for de novo selection or for example for an affinity maturation library on a select TCL14 target specific hit.

TABLE 4 Amino acid positions SEQ (in SEQ Amino acid ID Region ID NO: 27) sequence NO: A strand  1-12 LPAPKNLVVSRV 29 AB loop 13-16 TEDS 30 B strand 17-21 ARLSW 31 BC loop 22-28 TAPDAAF 32 C strand 29-37 DSFLIQYQE 33 CD loop 38-43 SEKVGE 34 D strand 44-50 AIVLTVP 35 DE loop 51-54 GSER 36 E strand 55-59 SYDLT 37 EF loop 60-64 GLKPG 38 F strand 65-74 TEYTVSIYGV 39 FG loop 75-81 KGGHRSN 40 G strand 82-89 PLSAIFTT 41 C strand + CD loop 29-43 DSFLIQYQESEKVGE 42 F strand + FG loop 65-81 TEYTVSIYGVKGGHRSN 43 A strand + AB  1-28 LPAPKNLVVSRVTEDSA 44 loop + B strand + RLSWTAPDAAF BC loop

In contrary to existing FN3-scaffold based library designs (Koide, et al., J Mol Biol, 284, 1141-1151, 1998; Koide et al., Proc Natl Acad Sci USA 104, 6632-6637, 2007; Dineen et al., BMC Cancer, 8, 352-361, 2008; Olson and Roberts, Protein Sci, 16, 476-484, 2007; Xu et al., Chemistry & Biology, 9, 933-942, 2002; Karatan et al., Chem Biol 11, 835-844, 2004; Hackel et al., J Mol Biol, 401, 84-96, 2010; Hackel et al., J Mol Biol 381, 1238-1252, 2008; Koide et al., Proc Natl Acad Sci USA, 104, 6632-6637, 2007; Lipovsek et al., J Mol Biol, 368, 1024-1041, 2007; Intl. Pat. Pub. No. WO2009/133208; Intl. Pat. Pub. No. WO2009/058379; U.S. Pat. No. 7,115,396), the designed TCL14 library surface has no similarity in structure to that of antibody variable domains or CDRs, or previously described FN3 libraries. Due to the large interaction surface generated by this design, high affinity molecules can be isolated quickly, possibly without the need for affinity maturation steps. Because this design does not

TABLE 5 Amino acid positions SEQ (in SEQ ID Amino acid ID Region NO: 28) sequence NO: C strand 29-37 DSFXIXYXE 45 F strand 65-74 TEYXVXIXGV 46 C strand + CD loop 29-43 DSFXIXYXEXXXXGE 47 F strand + FG loop 65-81 TEYXVXIXGVKGGXXSX 48 A strand + AB  1-28 LPAPKXLXVXXVXXXXAX 49 loop + B strand + LXWXAPDAAF BC loop E strand 55-59 XYXLT 50 randomize long stretches of consecutive amino acids, it may produce FN3 binding molecules that are more soluble and stable than previously described libraries. The TCL14 library described produces a flat or concave interaction surface in comparison to the curved surface of previous libraries. Thus, FN3 molecules selected from TCL14 are likely to bind to distinct antigens and epitopes as those found from previous FN3 library designs. The TCL14 library design may also allow for the production of two distinct binding surfaces on the same molecule to achieve bi-specificity. Generation of the TCL14 Library

The TCL14 library described above was expressed using the cis-display system (Odegrip et al., Proc Natl Acad Sci USA 101: 2806-2810, 2004). In this system, the library is ligated to DNA fragments encoding the RepA coding sequence, cis and ori elements, and a Tac promoter, and the resulting ligation product is in vitro transcribed/translated. The produced TCL14-RepA fusion proteins are bound in cis to the DNA by which the fusion proteins are encoded. The library is screened for scaffold molecules binding specifically to proteins of interest, the molecules are isolated and the bound DNA amplified to identify the coding sequences of the bound scaffold molecules.

TCL14 library was generated by randomizing positions L32, Q34, Q36 (C-strand), S38, E39, K40, V41 (CD-loop), T68, S70, Y72 (F-strand), H78, R79, and N81 (FG-loop) in Tencon 27 (SEQ ID NO: 27) using the polymerase chain reaction (PCR) with degenerate primers and cloned 5′ to the RepA gene for cis-display using standard protocols. The primer C-CD N46V (SEQ ID No. 51) was used to randomize the C strand and the C:D loop and the primer F-FG-Sf E86I-R (SEQ ID No. 52) was used to randomize the F strand and the F:G loop. The final ligation was amplified with the primers R1RecFor (SEQ ID NO: 53) and DigLigRev (SEQ ID NO: 54) to generate the TCL14 library for in vitro transcription/translation. Table 6 shows the sequences of the primers utilized. Codon NNS were used for diversification (IUB code; N indicating A, C, G, or T; S indicating C or G).

TABLE 6 SEQ ID Primer Name Sequence NO C-CD N46V GCGGCGTTCGACTCTTTCNNSATCNNSTACNNSGAANNSNNSNNSNNSG 51 GTGAAGCGATCGGTCTGACCGTTCCGGGTTCTGAACGTTCTTACGACCT GACCGGTCTGAAACCGGGTACCGAATAC F-FG-Sf E86I-R GGTGGTGAAGATCGCAGACAGCGGSNNAGASNNSNNACCACCTTTAAC 52 ACCSNNGATSNNAACSNNGTATTCGGTACCCGGTTTCAGACCGGTCAGG TCGTA R1RecFor GAACGCGGCTACAATTAATACATAACC 53 DigLigRev CATGATTACGCCAAGCTCAGAA 54 TCON6 AAGAAGGAGAACCGGTATGCTGCCGGCGCCGAAAAAC 55 TCON5 E86I GAGCCGCCGCCACCGGTTTAATGGTGATGGTGATGGTGACCACCGGTG 56 short GTGAAGATCGCAGACAG Characterization of the TCL14 Library

The generated TCL14 library was PCR cloned into a modified pET15 vector (EMD Biosciences) containing a ligase independent cloning site (pET154-LIC) using TCON6 (SEQ ID NO: 55) and TCON5 E86I short (SEQ ID NO: 56) primers, and the proteins were expressed as C-terminal His6-tagged proteins after transformations and IPTG induction (1 mM final, 30° C. for 16 hours) using standard protocols. The cells were harvested by centrifugation and subsequently lysed with Bugbuster HT (EMD Chemicals, Gibbstown, N.J.) supplemented with 0.2 mg/mL Chicken Egg White Lysozyme (Sigma-Aldrich, St. Louis, Mo.). The bacterial lysates were clarified by centrifugation and the supernatants were transferred to new 96 deepwell plates. The proteins were purified using a 96 well Ni-NTA Multitrap Plate (GE Lifesciences, Piscataway, N.J.).

A random selection of clones was picked and sequences to evaluate obtained distribution in the library. The observed diversity in the library was well in accordance to the expected (FIG. 7). To calculate the observed diversity in the full length library clones, the total number of times a given amino acid appeared in the diversified library regions was counted in all clones and divided by the total number of random positions (13 random library positions * 69 full length clones) and multiplied by 100 to yield % Frequency. The Expected Diversity is based on the NNS degenerate codon with the following amino acid distribution: Phe=n1, Leu=3, Ile=1, Met=1, Val=2, Ser=3, Pro=2, Thr=2, Ala=2, Cys=1, Arg=3, Gly=2, Tyr=1, His=1, Gln=1, Asn=1, Lys=1, Asp=1, Glu=1, Trp=1 codon(s) divided by the total number of codons (32) multiplied by 100 to yield % Frequency.

Purified proteins were subjected to size exclusion chromatography to determine the aggregation propensity of individual library members. The elution profiles of select clones were determined by injecting 10 μL of the purified proteins onto a Superdex 75 5/150 column using an Agilent 1200 HPLC with absorbance read at 280 nm. ˜80% of the non-cysteine containing clones eluted as a single, monomeric peak, thus signifying that the majority individual library members have retained the intrinsic solubility and structure of the parent molecule. Some molecules containing free cysteine were found to oxidize after purification and thus elute as dimeric molecules.

Differential Scanning calorimetry (DSC) was used to further characterize clones that had a monodispersed profile as determined by SEC analysis. DSC data was obtained by heating 0.5 mg/mL solutions for each clone in PBS from 35° C. to 95° C. at a rate of 1° C./min in a VP-DSC capillary cell microcalorimeter (Microcal, LLC, Piscataway, N.J.). Melting temperatures were calculated for each clone using CpCalc (Microcal, LLC, Piscataway, N.J.) software with a summary of the data shown in Table 7. The average melting temperature of the tested molecules was 70±9° C. The obtained data demonstrates that the TCL14 library design produces scaffold molecules that have retained a significant amount of the thermal stability of the parent molecule Tencon25 (93° C.) and are themselves inherently thermally stable and well folded.

TABLE 7 Clone Number Tm (° C.) TcCF-003 60 TcCF-004 61.5 TcCF-006 76.3 TcCF-031 71.2 TcCF-041 71 TcCF-078 68 TcCF-082 87 TcCF-083 72.3 TcCF-084 62.3 TcCF-090 70.2 TcCF-092 71.5 TcCF-103 51 TcCF-106 87.3 TcCF-107 74.5 TcCF-111 68 Selection of TCL14 Library Molecules Specifically Binding to Target Molecules of Interest

The TCL14 library was screened against various target proteins of different protein classes consisting of cell surface receptor extracellular domains, cytokines, kinases, phosphatases, heat shock proteins and immunoglobulins and their fragments to identify scaffold molecules specifically binding to these proteins and/or protein domains. Purified soluble proteins expressed in HEK293 or E. coli cells were biotinylated using the EZ-Link No-Weigh Sulfo-NHS-LC-Biotin Microtubes (Thermo Fisher, Rockford, Ill.) followed by extensive dialysis into PBS. For selections, 3 μg of TCL14 library was in vitro transcribed and translated (IVTT) in E. Coli S30 Linear Extract (Promega, Madison, Wis.) and the expressed library blocked with Cis Block (2% BSA (Sigma-Aldrich, St. Louis, Mo.), 100 μg/ml Herring Sperm DNA (Promega, Madison, Wis.), 1 mg/mL heparin (Sigma-Aldrich, St. Louis, Mo.). For selection, each biotinylated target protein was added at concentrations of 400 nM (Round 1), 200 nM (Rounds 2 and 3) and 100 nM (Rounds 4 and 5). Bound library members were recovered using neutravidin magnetic beads (Thermo Fisher, Rockford, Ill.) (Rounds 1, 3, and 5) or streptavidin magnetic beads (Promega, Madison, Wis.) (Rounds 2 and 4) and unbound library members were removed by washing the beads 5-14 times with 500 μL PBST followed by 2 washes with 500 μL PBS.

Following 5 rounds of selection, the DNA output was amplified by PCR and subcloned into pET154-LIC using standard protocols.

Additional selection rounds were performed in order to identify scaffold molecules with improved affinities for two target proteins. Briefly, outputs from round 5 were prepared as described above and subjected to additional iterative rounds of selection with the following changes: incubation with biotinylated target protein was decreased from 1 hour to 15 minutes and bead capture was decreased from 20 minutes to 15 minutes, biotinylated target protein decreased to 25 nM (Rounds 6 and 7) or 2.5 nM (Rounds 8 and 9), and an additional 1 hour wash was performed in the presence of an excess of non-biotinylated target protein. The goal of these changes was to simultaneously select for binders with a potentially faster on-rate and a slower off-rate yielding a substantially lower K_(D). The 9^(th) round output was PCR amplified, cloned and expressed as described above.

In Vitro Characterization of Scaffold Molecules Binding to Proteins and/or Protein Domains of Interest

Binding

Enzyme linked immunosorbant assay (ELISA) was performed on 188 individual clones from the round 5 panning outputs. Maxisorp plates (Nunc, Rochester, N.Y.) were coated with 0.1 μg anti-His antibody (Qiagen, Valencia, Calif.) overnight, washed with Tris-Buffered Saline, pH 7.4 with 0.05% Tween-20 (TBST) and blocked using Starting Block T20 (Thermo Fisher, Rockford, Ill.). Clarified bacterial lysates containing 1 μg/ml His₆-tagged TCL14-RepA fusions or a control protein (human serum albumin) were applied onto the wells of the coated plates. The plates were incubated for 1 hour, washed with TBST and the biotinylated protein detected with streptavidin-HRP (Jackson Immunoresearch, West Grove, Pa.) and POD chemiluminescent substrate (Roche, Indianapolis, Ind.) using Molecular Devices M5 plate reader. Performance of the library was assessed by a hit rate. The hit rate was defined as the percent (%) of scaffold molecules having 10-fold luminescence signal above the control signal divided by the total number of clones screened (188). As shown in Table 8, the TLC14 library yielded scaffold molecules with hit rates ranging between 8% to 45% for eight distinct proteins. Cytokine 2 is mouse IL-17A.

TABLE 8 Target Hit Rate (%) Ser/Thr Kinase 37 Receptor ECD 45 Immunoglobulin 22 Heat Shock Protein 18 Cytokine 6 Immunoglobulin 2 42 Cytokine 2 18 Phosphatase 8 Characterization of Mouse IL-17A Binders IL-17A Receptor Inhibition

An inhibition assay was performed to determine if the round 5 and 9 panning outputs against mouse IL-17A (mIL-17A) inhibited binding of mIL-17A to the mIL-17A receptor. Maxisorp plates were coated with 0.2 μg/ml mIL-17A receptor Fc fusion (R&D Systems, Minneapolis, Minn.) overnight, washed with Phosphate-Buffered Saline (PBS), pH 7.4 with 0.05% Tween-20 (TBST) and blocked with 2% BSA, 5% Sucrose in PBS. 10 ng/ml biotinylated-mIL17A (b-mIL-17A) was added into the clarified bacterial lysates diluted 1:50 in 1% BSA in PBS, and the mixtures were incubated for 20 minutes. The blocked plates were washed and the bacterial lysates/b-mIL-17A incubations were transferred onto the plates. The plates were incubated for an additional hour, washed with PB ST, and the biotinylated protein detected with streptavidin-HRP (Jackson Immunoresearch, West Grove, Pa.) and OPD colorimetric substrate (Sigma-Aldrich, St. Louis, Mo.). Absorbance at 490 nm was read using an M5 plate reader (Molecular Devices, Sunnyvale, Calif.) and the data converted to % inhibition. Percent inhibition for mIL-17A:mIL-17 receptor binding was defined as 100−(sample/negative control×100).

Select bacterial lysates containing the scaffold molecules inhibiting the mIL-17A:mIL-17 receptor interaction were further characterized in a dose response inhibition assay using the protocol described above, except that 100 μl of purified TCL14-His (Ni-NTA) fusion proteins were used in the assays between concentrations of 10 μM to 56 pM. IC50 values were calculated from the dose response curves using a sigmoidal dose response fit. As summarized in Table 9, the mIL-17A specific inhibitors have a range of IC50s from ˜9 to ˜428 pM.

TABLE 9 Clone ID IC₅₀ (pM) k_(on) (1/Ms) k_(off) (1/s) K_(D) (M) TP1KR9P61-A2 33.93 137000 3.93E−05 2.87E−10 TP1KR9P61-A7 55.75 82000 3.46E−05 4.21E−10 TP1KR9P61-E2 42.82 147000 3.96E−05 2.70E−10 TP1KR9P61-G4 8.83 162000 5.02E−05 3.09E−10 TP1KR9P62-A2 261.1 408000 2.17E−05 5.31E−11 TP1KR9P62-C3 117.1 281000 1.05E−05 3.74E−11 TP1KR9P62-C6 109.1 568000 1.20E−05 2.12E−11 TP1KR9P62-D3 91.18 110000 6.07E−05 5.54E−10 TP1KR9P62-D4 242 105000 1.00E−05 9.52E−11 TP1KR9P62-D8 427.5 381000 1.48E−05 3.89E−11 TP1KR9P62-E3 64.16 113000 5.26E−05 4.64E−10 TP1KR9P62-H10 301.8 438000 2.11E−05 4.82E−10 Affinity Measurements

The affinities of select molecules binding to mIL-17A were measured using surface Plasmon resonance using a ProteOn XPR-36 instrument (Bio-Rad). Purified molecules were directly immobilized on the chip via amine coupling with varying densities (100˜300 Rus) at pH 5.0 and a flow rate of 30 μL/min for 5 minutes mIL-17A at 100 nM diluted in a 3-fold concentration series was tested for their binding to different molecules on the chip surface. The dissociation phases for all concentrations of all samples was monitored for 1˜2 hours at a flow rate of 100 μL/min depending on their off-rate. A buffer sample was injected to monitor the baseline stability and the surface was not regenerated for further use. The response data for all concentration series for each of the different surfaces of the scaffold molecules selected from the TLC14 library were globally fit to a 1:1 simple langmuir binding model to extract estimates of the kinetic (k_(on), k_(off)) and affinity (K_(D)) constants. As summarized in Table 9, affinities of the scaffold molecules specifically binding mIL-17A were at a subnanomolar range. Sequences of select mIL-17A binders are shown in SEQ ID NOS: 85-96, and the sequences of the C and F beta-strands and the CD and the FG loops in Table 10.

TABLE 10 C strand CD loop SEQ ID SEQ ID Clone ID sequence NO: sequence NO: TP1KR9P61-A2 DSFAIEYFE 63 DWWSGE 67 TP1KR9P61-A7 DSFAIEYFE 63 DWWSGE 67 TP1KR9P61-E2 DSFAIEYFE 63 DWWSGE 67 TP1KR9P61-G4 DSFAIEYFE 63 DWWSGE 67 TP1KR9P62-A2 DSFAIEYSE 64 DYWLGE 68 TP1KR9P62-C3 DSFAIEYFE 63 DWWSGE 67 TP1KR9P62-C6 DSFAIEYFE 63 DWWSGE 67 TP1KR9P62-D3 DSFAIEYFE 63 DWWSGE 67 TP1KR9P62-D4 DSFGIIYFE 65 DWWAGE 69 TP1KR9P62-D8 DSFAIEYFE 63 DWWSGE 67 TP1KR9P62-E3 DSFGIEYFE 66 DYWTGE 70 TP1KR9P62-H10 DSFAIEYFE 63 DWWSGE 67 F strand FG loop SEQ ID SEQ ID Clone ID sequence NO: sequence NO: TP1KR9P61-A2 TEYAVSIRGV 71 KGGMPSA 75 TP1KR9P61-A7 TEYSVSIRGV 72 KGGYPSS 76 TP1KR9P61-E2 TEYAVSIRGV 71 KGGMPSP 77 TP1KR9P61-G4 TEYAVSIRGV 71 KGGYPSA 78 TP1KR9P62-A2 TEYGVSIRGV 73 KGGYPSP 79 TP1KR9P62-C3 TEYSVTIRGV 74 KGGPPSS 80 TP1KR9P62-C6 TEYSVTIRGV 74 KGGYPSS 81 TP1KR9P62-D3 TEYSVSIRGV 72 KGGYPSS 81 TP1KR9P62-D4 TEYGVSIRGV 73 KGGPPSR 82 TP1KR9P62-D8 TEYGVSIRGV 73 KGGLASP 83 TP1KR9P62-E3 TEYAVSIRGV 71 KGGYPSA 78 TP1KR9P62-H10 TEYSVSIRGV 72 KGGHPSV 84

EXAMPLE 4 Tencon27 Libraries Randomized at a Second Alternative Surface

A second alternative surface on Tencon27 resides on the opposite side of the C-CD-F-FG surface as visualized in FIG. 2C, herein called the A-AB-B-BC-E surface, formed by the A beta-strand, the AB loop, the B beta-strand, the BC loop, and the E beta-strand. The A-AB-B-BC-E surface is also slightly concave, and can be used as a template to design libraries of protein scaffold interaction surfaces by randomizing a subset of residues that form the surface. Beta-strands have a repeating structure with the side chain of every other residue exposed to the surface of the protein. Thus, a library can be made by randomizing some or all surface exposed residues in the beta-strands. By choosing the appropriate residues in the beta-strands, the inherent stability of the Tencon27 scaffold should be minimally compromised while providing a unique scaffold surface for interaction with other proteins. Randomizing the A-AB-B-BC-E surface will produce a binding surface on the opposite side of the Tencon27 structure when compared to the TCL14 library design. The library design on Tencon27 with randomized A-AB-B-BC-E surface is shown in SEQ ID NO: 61 (the TCL15 library) and in FIG. 6.

TCL15 library (SEQ ID NO: 61): LPAPKXLXVXXVXXXXAXLXWXAPDAAFDSFLIQYQESEKVGEAIVLTVP GSERXYXLTGLKPGTEYTVSIYGVKGGHRSNPLSAIFTT; wherein X is any amino acid.

The TCL15 library is generated and selected for scaffolds specifically binding target molecules as described above for the TCL14 library.

EXAMPLE 5 Other FN3 Domains: Generation of Libraries by Randomizing Alternative Surfaces

The library designs utilizing alternative surfaces described in the examples for the Tencon27 scaffold can be applied to other FN3 domains of various proteins due to the structural similarity among the FN3 domains. Such FN3 domains may be naturally occurring or synthetic, and are for example a Fibcon consensus scaffold (SEQ ID NO: 58) based on a consensus sequence of fibronectin domains (U.S. Pat. Pub. No. 2010/0255056), the 10^(th) FN3 domain of human fibronectin (FN10) (SEQ ID NO: 97), or the 3^(rd) FN3 domain from human tenascin (TN3) (SEQ ID NO: 3), or any FN3 domain present in proteins listed in Table 1.

Library designs for Fibcon, FN10 and TN3 libraries with randomized C-CD-F-FG alternative surfaces is shown in FIG. 8 and in SEQ ID NOS: 62, 98, and 99, respectively. Designed libraries are synthesized, expressed and selected for specific binders using protocols described within.

Fibcon-based protein scaffold library with randomized C-CD-F-FG surface (SEQ ID NO: 62):

LDAPTDLQVTNVTDTSITVSWTPPSATITGYXIXYXPXXXXGEPKELTVP PSSTSVTITGLTPGVEYXVXLXALKDNXXSXPLVGTQTT; wherein X is any amino acid. FN10-based protein scaffold library with randomized C-CD-F-FG surface (SEQ ID NO: 98):

VSDVPRDLEVVAATPTSLLISWDAPAVTVRYYXIXYXEXXXXSPVQEFTV PGSKSTATISG LKPGVDYXIXVXAVTGRGDSPXXSXPISINYRT; wherein X is any amino acid. TN3-based protein scaffold library with randomized C-CD-F-FG surface (SEQ ID NO: 99):

DAPSQIEVKDVTDTTALITWFKPLAEIDGIXLXYXIXXXXGDRTTIDLTE DENQYSIGNLKPDTEYXVXLXSRRGDXXSXPAKETFTT; wherein X is any amino acid.

Similarly to as described for the Tencon27 scaffold, some or all of the residues comprising the CD and/or FG loops of other FN3 domains can be replaced with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or 13 randomized positions to generate libraries of different lengths.

It will be clear that the invention can be practiced otherwise than as particularly described in the foregoing description and examples. Numerous modifications and variations of the present invention are possible in light of the above teachings and, therefore, are within the scope of the appended claims. 

What is claimed:
 1. A method of obtaining a protein scaffold that specifically binds to a target molecule, comprising: a. contacting or panning a library of fibronectin module of type III (FN3) domains having a diversified C-CD-F-FG alternative surface formed by a C beta-strand, a CD loop, an F beta-strand, and an FG loop with the target molecule, and b. isolating a protein scaffold specifically binding to the target molecule with a predefined affinity, wherein the library is formed by providing a reference FN3 domain polypeptide having the amino acid sequence at least 80% identical to that of SEQ ID NO: 27 and introducing diversity into the reference FN3 domain polypeptide by mutating at least one C beta-strand residue and at least one F beta-strand residue to form the FN3 domain library having the diversified C-CD-F-FG alternative surface, wherein the at least one residue in the C beta-strand is mutated with the proviso that S30 is not mutated (residue numbering according to SEQ ID NO: 27), wherein the at least one residue in the F beta-strand is mutated with the proviso that E66 is not mutated (residue numbering according to SEQ ID NO: 27), wherein each of the C beta-strand residues L32, Q34 and Q36 are mutated (residue numbering according to SEQ ID NO: 27) or each of the F-beta strand residues T68, S70 and Y72 are mutated (residue numbering according to SEQ ID NO: 27).
 2. The method of claim 1, wherein 1, 2, 3 or 4 residues in the CD loop residues in the reference FN3 domain polypeptide are mutated with the proviso that G42 and E43are not mutated (residues numbering according to SEQ ID NO: 27).
 3. The method of claim 2, wherein the residues S38, E39, K40 and V41 in the CD loop in the reference FN3 domain polypeptide are mutated.
 4. The method of claim 3, wherein 1, 2, 3 or 4 residues in the FG loop in the reference FN3 domain polypeptide are mutated with the proviso that the residues K75, G76, G77 and S80 are not mutated (residue numbering according to SEQ ID NO: 27).
 5. The method of claim 4, wherein the residues H78, R79 and N81 in the FG loop in the reference FN3 domain polypeptide are mutated (residue numbering according to SEQ ID NO: 27).
 6. The method of claim 5, wherein the reference FN3 domain polypepetide comprises an amino acid sequence of SEQ ID NO: 27, optionally comprising at least one substitution at amino acid positions 11, 14, 17, 37, 46, 73, or
 86. 7. The method of claim 6, wherein the reference FN3 domain polypeptide comprises an amino acid sequence of SEQ ID NO:
 28. 